Preparation of cyclohexane



April 10, 1945.- M. D. PETERSON 2,373,501

\ PREPARATION OF CYCLOHEXANE Filed April 18, 1942 CAMLysr am `v/' CVCLDHEXANE Bf/VZ ENE 7 f p BY K AATTORNEY Patented Apr. 10, 1945 v PREPARATION F CYCLOHEXANE Merlin DeweyPeterson, Edgemoor Terrace, Del.,

assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Dela- Ware Application April 18, 1942, Serial No. 439,534

(Cl. 26o- 667) I1 Claims.

This invention relates to the production of cyclohexane and more particularly to an improved process for its production by the catalytic hydrogenation of benzene.

In the manufacture of 'organic compounds, every'effort is made to lim-it the extent of side reactions and thereby potentially, at least. -increase the conversion to the 'desired product. Oxidation and hydrogenation reactions involving aromatic organic compounds are especially subject to side reactions with the concurrent decrease in yield of the products desired. Heretofore major attention has been directed, in general reactions of this nature, to the control of the` reaction rate and especially the control of the` temperature and pressure of. the reaction whereby some inhibition in the rate of side product formation has been realized. In spite of very accurate temperature and pressure control, however, many organic reactions of this nature have not been suiiiciently improved with respect to conversion rates'to advance their use beyond the academic stage.

An object of the present invention is to provide an improved process forthe hydrogenation of benzene to cyclohexane. invention is to provide conditions for the hydrogenation of benzene to cyclohexane whereby substantially no by-products of the reaction are obtained. Yet another object is to provide reaction conditions such that the above objects are attained. Still another object is to provide a process for the hydrogenation of benzene to cyclohexane whereby the benzene ows continuously through the reaction zone all portions of which are-not at the same temperature. Another ob- Anotherobject of the 'exothermic heat thereof is removed by condensation of the reactants vapozed throughout the reaction, whereby the necessity of -using complicated expensive converters is avoided.

Attention is directed to the single gure which diagrammatically illustrates a preferred embodiment of the invention by an examination of which the features of the invention can be readily appreciated. A suitable hydrogenation catalyst is disposed in the converter I (the disposition of the catalyst being more fully particularizedfhereinafter), which is prpvided with pressure sustaining walls. Through the inlet pipe 2, benzene is introduced into the converter which is lled to a level above the catalyst 'in order to insure that throughout the reaction the catalyst will remain totally submerged. Hydrogen is lntroduced into the converter through the manifold 3 from which the hydrogen is led through pipes 4 fitted with suitable jets 4 a which distribute the hydrogen uniformly into the base of the hydrogenation converter I from which the hydrogen is uniformly distributed into the catalyst bed by means of the perforated plate 5.

The lower section A of the converter I is charged with catalyst supported on the perforated plate 5 to hold the catalyst out of'direct contact ject is to provide a temperature differential within thereaction zone whereby exceptionally highyields of cyclohexane 4by hydrogenation of bento give substantially quantitative yields, together with efllcient utilization of hydrogen, if the reaction is carried out within a reaction zone which s maintained witha differential temperature between that portion of the zone Where the benzene concentration is high and that portion of the zone where the cyclohexan'e concentration is high. A further feature of .the invention involves' control of the temperature or the reaction' and more especially the differential temperatures bewith the hydrogen inlet jets 4a, the section being only partially filled to keep the catalyst below the temperature control coil 6. The catalyst in y the middle compartment B is supported on the screen 'I and is only partially lled so that a catalyst-free space. remains at the top for the temperature control coil Il. Similarly compartment C is only partially filled with catalyst which is supported on screen 9 for holding the catalyst in the compartment and out of immediate Contact with the temperature control coil 8. The liquid level, as has been stated, is maintained above the catalyst in compartment C but as shown is maintained well below the top of the converter. A vapor exit pipe I0 is provided at the top of the hydrogenation converter which conducts the unused hydrogen, inert gases, and `vapors driven from the reaction zone into a condenser I I which is provided with conventional cooling means entering and leaving the condenser through pipes l2 and I3 respectively. The condensate from the condenser II passes directly into the benzene supply pipe 2 and through said pipe is returned tothe hydrogenation converter I. The non-condensable vapors pass from the system through pipe I4. If desired, the vapors from pipe I4 may be reintroduced into the converter by connection for hydrogenation reactions generally and. particularly those which have a relatively high exothemic heat of reaction. When such organic compounds are hydrogenated the major portion of the hydrogenation takes place in the upper part ofthe converter and consequently the major portion of the heat resulting from the reaction is likewise produced in this part of the converter. When equilibrium conditions have been established and a balance of the system realized by regulating the flow of the compound to be hydrogenated into the converter concurrently with a regulation of the hydrogen input together with the hydrogenated lproduct discharged from. the converter, it will be found that the temperature within the reactant liquid will vary from a high at the top of the converter to a low at the bottom thereof. This temperature difference or gradient will be maintained and will remain subpossible, therefore, by means of this method of hydrogenation to employ at the top of a converter conditions for maximum rate of hydrogenation and toward the bottom of the converter conditions more favorable for substantially complete hydrogenation of the product. By a proper regulation of the temperature throughout the converter, therefore, it is possible to obtainisub'- stantially quantitative yields of the hydrogenated product, at reaction rates approaching those corresponding to the maximum converter temperature, but with amounts of excess hydrogenation approaching those corresponding to the more favorable equilibrium at the lower converter temperature.

The invention may be illustrated more speciilcally Iby the hydrogenation of benzene to cyclohexane. The converter is charged with a suitable catalyst and preferably a rugged catalyst having a mesh size or from 1 to 4. It has been found that a highly active catalyst for the reaction may be prepared from a nickel-aluminum alloy containing approximately 40% nickel and` 60% aluminum, which is crushed to size prior to being placed in the converter. Valves I6 are closed and through valves I'I and pipes I0 and I5 a hot dilute aqueous sodium hydroxide solution is passed until from,2025% of aluminum in the alloy has been removed. The hot sodium hydroxide solution is then displaced fromthe converter by the consecutive displacement with cold sodium hydroxide solution, cold water, methanol, and cyclohexane. Valves I'I are then closed. Byaneans of the heating -coils 6 and 8 the cyclohexane is heatd Within the reaction zone to between 150 and 200 C. Valves I6 are then opened and benzene is introduced through pipe 2 while simultneously hydrogen is introduced through the manifold 3, under a suitable pressure to build up a pressure in the converter to about 25 atmospheres. Hydrogen bubbles upwardly countercurrent to the descending benzene liquid and the exothermicity of the reaction is such that the temperature rises rapidly at the point of greatest reaction rate which is within the top section of 2,878,501 with the hydrogen manifold 3. The reactant and the converter. At this point the temperature under this pressure is at equilibrium in the neighborhood of 210220 C. Unconverted hydrogen passes from the surface of the liquid together with any inerts contained in the hydrogen used together with vaporized benzene or cyclohexane which mixture of permanent gases and vapors passes through pipe I0 into the condenser. In this condenser the benzene and cyclohexane are condensed, are passed into the benzene delivery tube 2 and from this tube flow directly into the top of the converter. Uncondensible gases in the vapors pass from the system through pipe I4. If these gases are high in hydrogen content they may be reintroduced through hydrogen manifold 3 into the converter. When the reaction has started a heating or cooling fluid is supplied, if necessary, through coils B and 8 in order to maintain the temperature at the proper amount. A temperature at the product end of the converter of 150 C. with the temperature of approximately 210 C. at the top of the converter gives excellent yields.

The ow of hydrogen into the converter should be atthe rate of at least 3 moles per mole of the benzene to satisfy stoichiometric requirements and as this is a r-eaction conducted preferably to equilibrium an excess of hydrogen over and above stoichiometric requirements should be used. For instance, greater than 99% conversion of the benzene to cyclohexane is obtained, under the above-described conditions, with a ratio of hydrogen to benzene on a molar basis of at least 3.5 to l (a 17% excess of hydrogen). `If, how-ever, equilibrium is establishedy at a temperature of 210 more than 5/1 molar hydrogen to benzene ratio for 99% conversion, or more than a 6'7 excess of hydrogen is required.

Although there will be some temperature gradient without cooling by means of coils 6 and A the catalyst and upiiowing hydrogen, and an exy cellent yield of cyclohexane from the benzene and hydrogen is thereby provided. The cyclohexane is discharged from the converter through pipe I5 and-will be found to contain in accord with thespecific conditions herein designated better than 99% cyclohexane.

The preferred embodiment `of the invention has been described utilizing a catalyst containing nickel and aluminum. Other suitable hydrogenation catalysts may be employed, however, such, for example, as nickel oxide, copper chromate, and mixtures of nickel, copper, and copper chromate, copper obtained by the reduction of copper oxide, nickel supported on various supports such as kieselguhr, silica gel', and the like, mixtures of copper, chromium, zinc, mixtures of zinc, and chromium, and in fact any lhydrogenation catalyst suitable for reactions of this nature.

The reaction has been illustrated as being conducted at a temperature in the proximity of 21o-220 C. It has been found, however, that the benzene to cyclohexane reaction occurs over a y which the reaction is conducted. In order to obtain the exceptional yields of reactants at rapid rates made possible by this invention the reactionbetween 180 `to 250 C. while the temperature in the bottom of the converter should preferably be below 180 C. and may be as low as room temperature. Moreover, the reaction proceeds favorably under elevated pressures and preferably at pressures between and 60 atmospheres although pressures from atmospheric pressure to 700 atmospheres may be employed if desired.

After the reaction has been initiated in the manner described a temperature differential is set up, as described, within the converter, inasmuch as the pure benzene'fiowing into the top of the converter rapidly reacts in the upper section thereof with hydrogen to give oil considerable heat of reaction. As the benzene now containing some cyclohexane passes downwardly into the converter the cyclohexane increases in concentration as the benzene decreases in concentration until at substantially the bottom of the converter there is no more benzene to be hydrogenated and there is found an approximatelyl 99% or better concentration of the cyclohexane.

Inasmuch as there is no benzene to be reacted reason that while the hydrogenation of benzene,'

as do most chemical reactions, proceeds more rapidly at higher temperatures, nevertheless, high yields of reactants are favored by lower temperatures of reaction and consequently thissystem of hydrogenation provides higher reaction rates during the initial stages of the reaction with lower reactionv rates and more favorable equilibrium conditions, which are suiicient to give substantially 100% conversion, during the latter stages of the reaction. y Moreover, by varying the pressure on the sysbenzeneithrough the apparatus. When operating in this manner, however, a greater amount of heat must be removed from the bottom of the converter by. for example,.extending the length, thereby losing heat through greater wall surface exposed, more elaborate cooling coils in this portion of the converter, or other means.

I claim:

1. A liquid phase process for the hydrogenation of benzene to cyclohexane wherein the benzene is continuously introduced into a reaction zone and the reaction mixture is continuously discharged from that zone which comprises initiating the hydrogenation of the benzene by contacting liquid benzene with a hydrogenation catalyst and passing the benzene countercurrent to the flow of hydrogen at an elevated temperature and gradually lowering the-contact temperature of the reacting mixture until a substantially quantitative yield of cyclohexane is obtained, the

gradual lowering of the contact temperature being accomplished with a maximum temperature tem, it is possible to raise or lower the temperature within the converter for example by increasing the pressure above 25 atmospheres as is designated in the preferred embodiment described, the amount of reactants being vaporized from the reaction zone decrease with an attendant decrease in the amount of condensable vapors entering the condenser and as a consequence the temperature of the reaction will be increased. Contrariwise by decrease in the pressu're lower temperatures are obtainable. 4 i

To providethisdesirable eifectof a differential temperature used during the reaction, it is advantageous to employ a vessel 4having greater length than cross section for as the length of thevessel approaches the cross sectional diameter thereof the temperature gradient in the liquid decreases until it is substantially non-existent. Accordingly, it is desirable to have aaratio of the length to the diameter of the vessel of at least l.5:1.

The apparatus has been described with particat the inlet ofthe reaction zone and a minimum temperature at the exit of the reaction zone.

2. A liquid phase process for the hydrogenation of benzene to cyclohexane wherein the benzene is continuously introduced into a reaction zone and the reaction mixture iscontinuously discharged from that zone which comprises initiating the vhydrogenation of the benzene by contacting liquid benzene with a hydrogenation cat'- alyst and passing the benzene countercurrent. to the flow of hydrogen at a temperature between 180 and 350 C., and' gradually lowering the contact temperature of the reacting mixture until a substantially quantitative yield of cyclohexane is obtained. the gradual lowering of the contact temperature being accomplished with a maximum temperature at the inlet of the reaction zone and a minimum temperature at the exit of the reaction zone.

3. A liquid phase process for the hydrogenation of benzene to cyclohexane wherein the benzene is continuously introduced into a reaction zone and the reaction mixture is continuously discharged from that zone which comprises'initiating the hydrogenation of the benzene by contacting liquid benzene with a hydrogenation catalyst and passing the benzene countercurrent to the flow of hydrogen at a temperature between 180 and 225 C. and at a pressure between 10 and 60 atmospheres and terminating the reaction Azene is continuously introduced into'a reaction zone and the reaction mixture is continuously disular reference to carryingvout the reaction with the countercurrent flow of benzene and hydrogen.

Inasmuch as the cooling effect of the hydrogen charged from that zone which comprises initiating the hydrogenation of the benzene by contacting liquid benzene with a hydrogenation catalyst and passing it countercurrent to the flow of hydrogen at a temperature of approximately 210 C. and at a pressure of approximately 25 atmospheres and terminating the reaction at a temperature of approximately C., the reaction 'being conducted by gradually lowering the temperature from the maximum temperature of about 210 C. at the inlet of the reaction zone to a minimum temperature of about 150 C. at the exito: the reaction zone.

5. A liquid process for the1 hydrolxenatlon of benzene to cyclohexane wherein the benzene is continuously introduced into a reaction zone and the reaction mixture is continuously dis' charged from that zone whiclr comprises initiating the reaction by contacting liquid benzene with a hydrogenation catalyst, passing liquid benzene countercurrent to and in contact with hydrogen at a. temperature below the boiling point of the reaction mixture, under the reaction conditions being employed, and thereafter gradually lowering the temperature of the reacting mixture until substantially all of the benzene has been hydrogenated to cyclohexane, the gradual loweri ing of the contact temperature beingjaccompiished with a maximum temperature at the inlet of the reaction zone and a minimum temperature at the exit oi the reaction zone.\

6. A liquid phase process for the catalytic hydrogenation of benzene to cyclohexane wherein the benzene is continuously introduced into a reaction zone and the reaction mixture is continuously discharged from that zone which comprises initiating the reaction by contacting liquid benzene with a hydrogenation catalyst and passing the benzene countercurrent to and in contact with hydrogen at the highest temperature at which the benzene is hydrogenated and terminatingr the reaction by contacting the Partially hydrogenated reaction product with hydrogen and a hydrogenation catalyst at the lowest temperature at which the benzene is hydrogenated, whichv temperature is lower than the temperature at which the hydrogenation is initiated, the temperature being controlled by the latent heat of evaporation ofthe reaction mixture.

7. A liquid phase process for the catalytic hydrogenation of benzene to cyclohexane which comprises introducing liquid benzene into the upper portion of a hydrogenation converter having a ratio of length to cross section of at least 1.5 to 1 and into contact with a hydrogenation catalyst therein at a temperature between 100 and 350* C. and' at elevated pressures the liquid benzene flowing counter-current to the ilow of hydrogen, and thereafter introducing the partially hydrogenated reaction mixture into contact with a hydrogenation catalyst and into another portion of the converter wherein the hydrogenation is conducted under a temperature at least 5 C.

'below the initial reaction temperature the reaction being conducted by gradually lowering the temperature from the maximum temperature-at l the beginning of the reaction to the minimum temperature atthe end of the reaction. i

8. A liquid phase process ior thecatalytic 1131- drogenation of benzene to cyclohexane which comprises initiating the reaction by introducing liquid benzene into contact with a hydrogenation catalyst, countercurrent to the flow of hydrogen, and into a reaction zone maintained at a temperature between 180 and 250 C'. and at a the beginning of the reaction to the minimum v temperature at the end of the reaction.

9. The process of claim 1 wherein the temperature differential is between 30 and 175 C.

10. The process of claim 1 wherein a temperature diil'erential is maintained by' evaporation of benzene and cyclohexane vapors :from the reaction mixture, condensing them and, returning them to the reaction mixture.

11. The process of claim 1 wherein a temperature differential between 30 and 175 C. is maintained by evaporation of benzene and cyclohexane vapors from the reaction mixture, condensing them and returning them to the reaction mixture.

MERLIN DEWEY PETERSON. 

